1. Field of the Invention
The present invention relates to a power transmission mechanism provided in order to absorb, buffer, or cut off an excessive torque when the torque is transmitted. More particularly, the present invention relates to a power transmission mechanism suitably used as a part that transmits torque from a pulley to the rotating shaft of a refrigerant compressor in a belt transmission device for rotatably driving a refrigerant compressor of an air conditioner for a vehicle.
2. Description of the Related Art
A prior art reference relating to this type of power transmission mechanism is Japanese Unexamined Patent Publication (Kokai) No. 2003-35255 (JP 2003-35255). Important parts of a power transmission mechanism described in (JP 2003-35255) are shown in FIG. 3. There may be a case where the general configuration except the important parts is similar to that according to the prior art described in (JP 2003-35255), when a concrete embodiment of a power transmission mechanism according to the present invention is described in detail later. The prior art is described here with reference to FIG. 3.
In FIG. 3, reference numeral 1P denotes a rotating shaft of a refrigerant compressor, which is rotatably driven by a pulley (not shown) on an engine via a belt. A hub 5P is attached to a driven pulley (not shown in FIG. 3). A part of the hub 5P makes up a so-called torque limiter and the whole of the hub 5P is made of a sintered metal, and an outer part (not shown in FIG. 3) and an inner part of the hub 5P are connected by a spoke-shaped narrow part (not shown) that breaks readily. On the inner surface of a cylindrical part 5Pc of the hub 5P integrally formed together with the inner part 5Pc, a female screw 5Pd is formed and is screwed into a male screw 1Pa formed on the outer surface at the end of the rotating shaft 1P. The direction of pitch of the screws 1Pa and 5Pd is formed such that the cylindrical part 5Pc is screwed and moves in the rightward direction in the figure along the rotating shaft 1P, by turning the driven pulley and the hub 5P in a direction in which they rotatably drive the rotating shaft 1P of the refrigerant compressor in a predetermined direction of rotation, assuming the rotating shaft 1P stays still.
The rotating shaft 1P is provided with a sleeve 6P attached thereto by means of insertion under pressure and a flat end surface 6Pa of the sleeve 6P is in contact with a flat end surface 5Pe of the cylindrical part 5Pc of the hub 5P. In addition, a tapered surface 6Pb formed on a part of the inner surface of the sleeve 6P is in contact with a tapered surface 1Pb formed on a part of the rotating shaft 1P. The prior art described in (JP 2003-35255) is characterized in that torque can be transmitted without fail between the hub 5P and the rotating shaft 1P, because it is possible to increase the area of a bearing surface (the tapered surfaces 1Pb and 6Pb, or the flat end surfaces 5Pe and 6Pa) interposed between the hub 5P and the rotating shaft 1P by attaching the sleeve 6P to the rotating shaft 1P without increasing the diameter of the rotating shaft, therefore, buckling (collapsing under pressure) is more unlikely to occur on these bearing surfaces.
When the sleeve 6P is firmly integrated with the rotating shaft 1P by means of insertion under pressure, torque is transmitted from the sleeve 6P to the rotating shaft 1P without fail by means of both a pressure-contact part at which the inner surface of the sleeve 6P and the outer surface of the rotating shaft 1P, which is inserted with pressure thereinto, are pressed to come into contact with each other, and the part at which the tapered surfaces 1Pb and 6Pb, which are pressed against each other as one of the bearing surfaces, come into contact with each other. However, the magnitude of the torque transmitted, prior to the above-mentioned transmission, from the cylindrical part 5Pc of the hub 5P to the sleeve 6P, is determined depending on the degree of slip between the flat end surfaces 5Pe and 6Pa, each of which is a bearing surface with respect to the other end surface. In the case where a slip occurs, if the torque is increased, a screw-tightening force acting between the screws 1Pa and 5Pd is increased and the slip in the direction of rotation between the end surfaces 5Pe and 6Pa is also increased and, therefore, the hub 5P slightly moves along the rotating shaft 1P in the axial direction to the right in the figure because the female screw 5Pd rotates relatively with respect to the male screw 1Pa. Due to this, the tensile force in the axial direction at the root part of the male screw 1Pa is increased locally and, thus, the possibility is brought about that the rotating shaft 1P may be broken.